Hard disk drives (HDD) are becoming smaller while at the same time providing greater storage capacity. One reason for these advances is the more prevalent use of brushless direct current motors (BLDC motor) to rotate the HDD. Further yet, a BLDC motor may be a three-phase motor and be driven by pulse-width modulation (PWM). For example, U.S. Pat. No. 6,137,253, which is incorporated by reference, discloses driving each of three BLDC motor coils with a respective PWM signal that causes a sinusoidal (or approximately sinusoidal) current to flow through each of the coils. By causing phase shifted (by approximately 120°) sinusoidal currents to flow through the coils, the BLDC motor is driven with a constant or approximately constant torque. This may be desirable in an application, such as disk drive applications, where it may be desirable to reduce or eliminate torque ripple in the rotation of the motor and that which the motor is rotating (e.g., a disk).
In one application, BLDC motor coils are driven with what is called a MacDonald voltage wave form, which is a PWM waveform that is described in U.S. Pat. No. 6,137,253. The MacDonald voltage wave form, when used to drive the motor coils, causes sinusoidal currents to flow through the coils. This may be accomplished by using a drive circuit having two drivers that comprise a high-side driver and a low-side driver. Thus, the Macdonald waveform may be conditioned to hold, for each 120° portion of the electrical period, one of the high-side or low-side drivers (MOSFET transistors in one example) for one of the coils in an ON state. Holding the high-side or low-side driver in an ON state may significantly reduce the switching losses in each drive transistor, and thus may significantly reduce the power dissipated by the chip.
While it may be desired that the spindle differential phase-to-phase current waveform be as symmetrical as possible (sinusoid) in order to have a constant spindle torque, there is no requirement that the absolute spindle phase voltage be held to a specific voltage to achieve this. Therefore, driver designers use this freedom to improve the driver operation through various drive algorithms that drive each phase pair or drivers, which may lead to reducing the switching losses, for example.